This invention relates generally to Magnetic Resonance Imaging (MRI) systems, and more particularly, to a Radio-Frequency (FR) coils in such MRI systems.
Magnetic Resonance Imaging (MRI) utilizes hydrogen nuclear spins of the water molecules in the human body, which are polarized by a strong, uniform, static magnetic field of a magnet. This magnetic field is commonly referred to as B0 or the main magnetic field. The magnetically polarized nuclear spins generate magnetic moments in the human body. The magnetic moments point in the direction of the main magnetic field in a steady state, but produce no useful information if these magnetic moments are not disturbed by any excitation.
The generation of Nuclear Magnetic Resonance (NMR) signals for MRI data acquisition is accomplished by exciting the magnetic moments with a uniform Radio-Frequency (RF) magnetic field. This RF magnetic field is commonly referred to as the B1 field or the excitation field. The B1 field is produced in the imaging region of interest by an RF transmit coil that is usually driven by a computer-controlled RF transmitter with a power amplifier. During excitation, the nuclear spin system absorbs magnetic energy and the magnetic moments precess around the direction of the main magnetic field. After excitation, the precessing magnetic moments will go through a process of Free Induction Decay (FID), releasing their absorbed energy and returning to the steady state. During FID, NMR signals are detected by the use of a receive RF coil, which is placed in the vicinity of the excited volume of the human body.
The NMR signal is the secondary electrical voltage (or current) in the receive RF coil that has been induced by the precessing magnetic moments of the human tissue. The receive RF coil can be either the transmit coil operating in a receive mode or an independent receive-only RF coil. The NMR signal is used for producing MR images by using additional pulsed magnetic gradient fields, which are generated by gradient coils integrated inside the main magnet system. The gradient fields are used to spatially encode the signals and selectively excite a specific volume of the human body. There are usually three sets of gradient coils in a standard MRI system that generate magnetic fields in the same direction of the main magnetic field, and varying linearly in the imaging volume.
In MRI, it is desirable for the excitation and reception to be spatially uniform in the imaging volume for better image uniformity. In known MRI systems, the best excitation field homogeneity is usually obtained by using a “whole-body” volume RF coil for transmission. The “whole-body” transmit coil is the largest RF coil in the system. A large coil, however, produces lower signal-to-noise ratio (SNR or S/N) if it is also used for reception, mainly because of its greater distance from the signal-generating tissues being imaged. Because a high signal-to-noise ratio is very desirable in MRI, special-purpose coils have been used for reception to enhance the S/N ratio from the volume of interest. In practice, a well-designed specialty or special-purpose RF coil has the following functional properties: high S/N ratio, good uniformity, high unloaded quality factor (Q) of the resonance circuit, and high ratio of the unloaded to loaded Q factors. Additionally, the coil should be mechanically designed to facilitate patient handling and comfort, as well as to provide a protective barrier between the patient and the RF electronics.
Another known method to increase the SNR is by quadrature reception. In this method, NMR signals are detected in two orthogonal directions, which are in the transverse plane or perpendicular to the main magnetic field. The two signals are detected by two independent individual coils that cover the same volume of interest. With quadrature reception, the SNR can be increased by up to, for example, √2 over that of the individual linear coils.
Sensitivity Encoding (SENSE) is a technique for reducing imaging time, thereby increasing imaging speed. In the SENSE technique, the spatial sensitivity information provided by the coil elements of a multiple-coil array system in real space can be used to substitute for the information provided by the encoding gradient in the k-space. By skipping some k-space lines, thereby saving imaging time, and using the spatial sensitivity information provided by each of the coil elements, an artifact-free full field of view (FOV) image can be reconstructed. For example, by eliminating two-thirds of the k-space lines (e.g., tripling the distance between two adjacent k-space lines), the imaging time may be reduced by about two-thirds (e.g., reduction factor=3).
Tripling the distance between two adjacent k-space lines also will result in a reduction of FOV in the imaging space to one-third of its original full FOV size. Therefore, the image intensity of each pixel inside the reduced FOV image will be the superposition of the image intensity of three pixels at three different locations in the full FOV image. With information about the spatial sensitivity profile of each coil element of a multiple-coil array system (at least three coil elements are needed) in the full FOV image and information relating to the forming of the reduced FOV image, the superimposed intensities can be separated for each pixel inside the reduced FOV image by solving a set of linear equations. Transferring the separated intensities of the three pixels back to their original locations and performing the same procedures for all the pixels inside the reduced FOV image results in a reconstructed original full FOV image. In order to perform SENSE imaging, the coil elements of an array coil system must distribute along the phase encoding directions.
In MRI and Magnetic Resonance Angiography (MRA), a neurovascular RF coil is typically used as a general-purpose coil for the head, neck/c-spine and vascular imaging without repositioning a patient. The coverage of a neurovascular coil, depending on the usable imaging volume (e.g., a sphere of forty-five to fifty centimeters (cm) in diameter) of known MRI systems, is about forty-eight cm from the top of the head to the aortic arch. The performance (e.g., SNR) and image uniformity of a neurovascular coil should be comparable to a conventional head coil for head imaging and to a stand-alone neck coil for neck/c-spine imaging. For vascular imaging, a neurovascular coil should be able to provide homogeneous images for coverage of the blood vessels from the Circle of Willis to the aortic arch. For head and vascular SENSE imaging, the sensitivity encoding needs to be performed in all the three directions, and specifically, in the left-right, anterior-posterior and superior-inferior directions.
To cover the head and neck with a single RF coil, an asymmetric birdcage coil is known. This coil includes anterior and posterior parts of a typical birdcage head coil, but is extended further over the neck and chest regions to provide coverage for these regions. The asymmetric birdcage coil is operated in quadrature mode for head and neck imaging. The enlargement of the birdcage head coil reduces the performance (e.g., SNR) of the head section of the asymmetric birdcage coil as compared to a conventional birdcage head coil. The anterior neck-torso coil section also is located a substantial distance from a patient's chest and the shape is often not optimized to fit the human neck-chest contour. Thus, the performance of the neck-torso section of the asymmetric birdcage coil is lower than that of the head section. The SNR drops quickly from the neck region to the chest region. This limits the coverage of the asymmetric birdcage coil, for example, to only the head and neck and not to the aortic arch.
It is also known to extend the coverage to the aortic arch using a quadrature RF coil for neurovascular imaging and spectroscopy of the human anatomy. This neurovascular coil utilizes multiple horizontal conductors and end conductors to distribute the current such that two orthogonal magnetic modes, and more particularly, one horizontal field and one vertical field, are created by the coil to achieve the quadrature detection of magnetic resonance signal. The neurovascular coil is separated into two shells, and specifically an upper shell for the anterior conductors and lower shell for the posterior conductors. These two shells may be connected by a hinge at the middle of the top end of the head coil mechanical housing. This coil arrangement may be used as a single coil for covering the entire FOV from the top of the head to the aortic arch. The anterior chest coil section also is attached to the anterior head coil and located a distance from a patient's chest. Thus, this neurovascular coil also results in lower performance, for example, lower SNR for the head imaging as compared to a conventional quadrature head coil and imaging non-uniformity of the chest region due to the quick SNR drop-off in this region.
Other coils arrangements are also known to allow imaging of a large field-of-view (FOV) while maintaining the SNR characteristic of a small and conformal coil. For example, a two-channel (four linear coils) volume array coil for magnetic resonance angiography of the head and neck is known. In this coil arrangement the first channel is a four bar quadrature head coil including two linear coils. Two Helmholtz type coils form the second channel for covering the neck and chest. The two Helmholtz type coils are arranged such that the magnetic fields generated are diagonally oriented and perpendicular to each other (i.e., a quadrature coil pair). The quadrature neck coil is attached to the quadrature head coil. Each of the two Helmholtz type neck coils overlap with the head coil to minimize the inductive coupling between the head and neck coils. The coverage of this two-channel quadrature volume array coil is limited to the head and neck and cannot image, for example, the aortic arch.
A split-top, four channel, birdcage type array coil also is known for head, neck and vascular imaging. This split-top head and neck coil includes a birdcage head coil and two distributed type (flat birdcage type) coils, one for the anterior neck-torso and the other for the posterior neck-torso. The quadrature signal obtained with the head coil is separated into two channels. The anterior and posterior neck-torso coils form the other two channels. The housing of the head and neck coil is divided into two parts, and specifically, a lower housing for the posterior one-half of the head coil and the posterior neck-torso coil and an upper housing for the anterior one-half of the head coil and the anterior neck-torso coil. The upper housing is removable providing a split top. Inductive coupling between the neck-torso coils and the head coil is minimized by overlapping the neck-torso coils with the head coil. The anterior neck-torso coil of the four channel vascular coil also is attached to the anterior head coil and located a distance from a patient's chest. Thus, a signal drop-off at the chest region results. Further, the decoupling of the multiple modes (i.e., multiple NMR frequencies) birdcage type anterior and posterior neck-torso coils from the multiple modes birdcage head coil is complex in design.
Neurovascular coils with a combination of a birdcage head coil and surface torso coils also are known. The performance of the head section of these neurovascular coils is lower than conventional standard birdcage head coils because of the design limitations. Further, other neurovascular coils are known and include multiple coils, for example, three volume-type coils and four or five surface coils. In these coil arrangements, two volume saddle coils are provided on a dome-shaped head coil former for brain imaging. Another two shaped saddle coils, one volume-type and the other surface-type, are used for the inferior portion of head and neck imaging. The torso section includes a loop-saddle quadrature pair for the posterior torso region and one or two loop coils for the anterior torso region. These coil arrangements again have design limitations.
These known coils not only have design limitations, but when used in SENSE operations, imaging in both the left-right (LR) and anterior-posterior (AP) directions is not possible in the head region. The complex sensitivity of the head coil elements does not allow for SENSE imaging to be performed.
SENSE imaging for neurovascular applications are known, such as a 16-channel neurovascular-SENSE array coil. This coil arrangement includes sixteen loop coils with eight bent loop coils for the head region and the other eight rectangular loop coils for the torso region. The eight head loop coils are constructed on a cylindrical former and tapered at the top of head region. The eight torso loop coils are separated into two sections with four for the anterior torso section and the other four for the posterior torso section. The four loop coils of each section are constructed on a planar former and arranged in the left-right direction. Each loop coil is separated from its adjacent coils by a gap and inductive coupling between adjacent coils is minimized using transformers. Thus, a 16-channel neurovascular-SENSE coil having a three-section arrangement is provided (e.g., head, anterior torso and posterior torso sections). However, signal drop-off may be experienced between the head and torso regions, which causes shading at the neck region. Further, as the number of coil elements increases for each of the head and torso regions, the size of each coil element decreases correspondingly. This further results in shading problems at the neck region. Additionally, this arrangement does not facilitate parallel imaging in the superior-inferior direction in the head region and in the torso region.